(1) Field of the Invention
The present invention relates to a process for the purification of dihydroxydiphenylsulfone. More particularly, the present invention relates to a purification process in which colorless dihydroxydiphenylsulfone is obtained from colored dihydroxydiphenylsulfone obtained by a reaction of phenol with sulfuric acid.
(2) Description of the Related Art
Dihydroxydiphenylsulfone has long been used as an additive to a plating solution, a leather tanning agent, a dyeing assistant, a curing promoter for a phenolic resin, a starting material for a flame retardant, an agricultural chemical and the like. Recently, however, dihydroxydiphenylsulfone has been used as a starting material for engineering plastics such as a polycarbonate, a polyester, and a polyether-sulfone, a starting material for a color photography element and a photographic contrast increaser, a color developer for a pressure-sensitive recording paper, and an additive to a vesicular recording material. In these applications, coloration of the dihydroxydiphenylsulfone must be reduced to a very low level, and the dihydroxydiphenylsulfone heretofore regarded as a purified product fails to satisfy this requirement.
Various processes have been proposed for obtaining dihydroxydiphenylsulfone having a reduced coloration. For example, a process comprising a treatment with a decoloring carbon is disclosed in Japanese Examined Patent Publication No. 38-5274, but a solution of a product treated by this process still has a light pink color. Japanese Examined Patent Publication No. 42-3005 discloses a process in which colored impurities are extracted and removed from a hot aqueous solution maintained at a temperature higher than 120.degree. C. by using an aliphatic higher alcohol. But, separation into an aqueous phase and an alcohol phase at a temperature higher than 120.degree. C. is not advantageous when carried out on an industrial scale. Furthermore, Japanese Examined Patent Publication No. 47-20223 discloses a process in which stoichiometric amounts of phenol and sulfuric acid are reacted at 130.degree. C. to 170.degree. C., benzene is added to the reaction mixture, and water formed by the reaction is removed by azeotropic distillation. Nevertheless, the dihydroxydiphenylsulfone is still colored brown and the hue to the product is not completely satisfactory. Japanese Examined Patent Publication No. 55-37550 discloses a process in which a reaction product is dissolved in an aqueous solution of phenol to separate the solution into two layers, and the lower layer containing oily impurities causing coloration is removed. In this process, an incorporation of phenol into the upper layer containing the purified product cannot be avoided, and this phenol becomes a new cause of coloration. Still further, Japanese Examined Patent Publication No. 58-29946 discloses a process in which dihydroxydiphenylsulfone containing a coloring substance is dissolved in a solvent and a reducing agent is added to the solution to effect decoloring, and Japanese Examined Patent Publication No. 58-42186 discloses a process in which a reaction product is dissolved in an aqueous solution of an alkali and the solution is treated with an inorganic peroxide. These processes, however, cannot produce a purified product having a satisfactory hue, and since the coloring impurity is merely rendered colorless by reduction or oxidation, the product is sometimes recolored if the product is used for a long time or under severe conditions.
In Japanese Unexamined Patent Publication No. 61-24559, the present inventors proposed a purification process comprising two steps of recrystallizing colored dihydroxydiphenylsulfone obtained by a reaction between phenol and sulfuric acid, from a methanol/water mixed solvent having a specific composition, and treating the recrystallization product with an activated carbon having a specific average pore diameter. In this process, however, two steps are necessary for removing coloring impurities. Japanese Unexamined Patent Publication No. 62-93271 discloses a process in which a basic aqueous solution of dihydroxydiphenylsulfone is brought into contact with an activated carbon, the activated carbon is removed by filtration or the like, and an acid is added to the residue to precipitate dihydroxydiphenylsulfone. In this process, however, a base and an acid must be used in amounts substantially equimolar to the dihydroxydiphenylsulfone, and therefore, they form salts and cannot be utilized again. Therefore, the process is not advantageous from the economical viewpoint.